Two top Republican senators are calling for a Senate Homeland Security Committee hearing on the Boston Marathon bombings last week, as lawmakers question whether enough was done to prevent the attack.

Sens. John McCain, R-Ariz., and Kelly Ayotte, R-NH, requested the hearing Wednesday, saying “it has become increasingly apparent that more questions need to be answered regarding the failure to prevent this tragedy.”

The senators cite a report by Fox News and others that Russian officials contacted the U.S. government at least twice in 2011 with concerns about Tamerlan Tsarnaev, the Chechen who two years later would carry out last week’s deadly bombing of the Boston Marathon, as an example of an instance that merits further investigation.

“In a string of apparent intelligence-sharing lapses, Tamerlan Tsarnaev was able to slip through the cracks and carry out this devastating attack,” the senators said.